In most buildings, there is a distribution system for water, normally with separate piping for cold and hot water. Often, water is provided from a public facility. For cold water, the water is lead directly from the source, possibly after reducing the pressure, and for the hot water, there is typically a local heater or a heat exchanger, so that cold and hot water can be tapped at various locations within the building.
From the source, the conduits are often quite long, so that fresh, relatively cold water will be obtained only after tapping quite a large volume of water that has been left in the conduits when water was tapped previously, maybe several hours or even days beforehand.
A similar problem is present for hot water, which will be cooled off when there is no longer any flow of hot water in the piping system from the heating source to the respective tap. In a prior art system, as disclosed in U.S. Pat. No. 5,944,221 (Laing), there is provided a re-circulation assembly. The cooled-down (previously hot) water in the hot water line is re-circulated by a pump back to the hot water tank, where it is reheated before being delivered again. Hereby, the heat energy contained in the hot water will be fed back to the hot water tank. However, this requires a special re-circulation piping system, which is generally rather costly to install and maintain. Because of the risk of growth of micro-organisms in a hot water line, it is not permitted to re-circulate the cooled-down water through the piping intended for cold and fresh water.